This invention relates to an electrode cap having an integral tank cover for acid mist collection. The acid mist collection to which this invention is applicable is utilized with electrochemical recovery or refining of metals, for example electrowinning of acidified copper from copper sulfate bearing solutions. The example now described relates to electrowinning of copper, although the concept can also apply to other metals and to electrorefining as well as electrowinning.

STATEMENT OF THE PROBLEM Processes utilizing electrolysis for the plating of metals are well known. What occurs is that in an electrolyte bath, metal is plated out from solution onto a cathode, sometimes concurrent with dissolution from an anode. In the case of electrowinning of copper from copper sulfate contained in solution with sulfuric acid, an exceptionally pure form of copper is extracted.

Oxygen gas is liberated at the, anode as a by-product of this electrolysis process. Unfortunately, this gas liberated during the process forms tiny bubbles which rise to the top of the plating bath. At the top of the plating bath, these bubbles burst. And when the bubbles — formed of thin layers of acid — burst, they emit to the surrounding atmosphere an acid aerosol. This acid aerosol is a source of pollution that has plagued electrowinning and electroplating.

Once the acid is in a mist, it is difficult to remove from the contaminated air except by utilizing processes involving the input of energy. Such processes include the utilization of large ventilation systems, scrubbers, precipitators or the like.

It will also be understood that the electrolyte has a vapor pressure. This vapor pressure also contributes to the

acid aerosol. This being the case, it will be understood that this disclosure is applicable to electrorefining. Likewise, this disclosure applies to permanent cathode technology and starter sheet technology. Variations can include other electrolytes other than sulfuric acid.

BACKGROUND OF THE INVENTION Attempts have been made in the prior art to remove and inhibit the acid mist arising over the tops of such plating tanks. In order to understand this aspect of the problem, a brief description of the electrowinning process for the reduction of copper interior of an electrolytic tank will be set forth. In the description of the process, the need to maintain ready access to the electrodes of the tank will be understood. Thereafter, a summary of the attempted solutions of the prior art will be set forth — together with their known shortcomings.

Modern electrowinning occurs in corrosion resistant tanks — typically made of plastic or plastic fiber concrete mixtures. These tanks are relatively large; they can be about 6 meters long, 1.2 meters across, and 1.4 meters deep, containing in the order of 8 cubic meters of electrolyte containing copper sulfate dissolved in a sulfuric acid solution. Each tank is provided with an array of depending typically flat electrodes. The electrodes are alternating planar cathode and anode electrodes suspended from the top of the tank and depending downward into the depth of the tank to a depth less than the total depth of the tank. The anodes are provided somewhere along their length with anode insulators; these insulators prevent direct anode to cathode shorting and maintain minimum anode/cathode spacing sufficient for the desired plating. Typically the cathodes, onto which the metal is plated, are larger than the anodes and provided with edge strips. These edge strips cause plating to occur only on the sides of the cathodes so that the copper when plated can conveniently be removed from the flat planar cathode surface. Provision is made for the inflow of fresh electrolyte at one

tank end and the outflow of depleted electrolyte at the opposite tank end.

Naturally, the electrodes are communicated with sufficient electrical current to cause the electroplating to occur. Consequently, bus connections to each tank combine to form electrical connections to each electrode resulting in the current between the electrodes to produce the required plating.

In the typical electrowinning process, the anodes are in large measure left in place. The cathodes must be periodically removed for the harvesting of the plated copper. Typically, the tanks are maintained as a group under a common roof in an otherwise large building referred to in the industry as a tank house. This imposes two practical requirements upon the tanks.

First, ready overhead access for the removal and insertion of the cathodes must be available. Second, the electrical connections — which are in a naturally corrosive environment — must be maintained in a relatively conductive state.

Having described the electrowinning environment this far, and remembering that the primary problem is the prevention of the escape of the acid mist, caused by the oxygen gas escaping during the plating process, the prior art attempts to alleviate this problem can now be set forth.

It has been realized in the industry that conventional covering of such tanks is not satisfactory. First, such covering interferes with the required ready access for the cells; removing and replacing a cover before cathode removal or other tank service is not acceptable. Secondly, the covering of the electrical connections to the anodes and cathodes is not acceptable. Corrosion and depositions under covers destroys conductivity and builds resistance. Finally, acid mist coalesces on the covers in a concentrated format. It then drips down onto the covered electrode supporting parts and connections of the tank, causing corrosion and shorting. As a consequence, for at least these reasons, such covers are not used.

The most commonly used expedient is voluminous ventilation. Massive amounts of air are circulated through such tank houses in the hopes that the acid mist can be swept away before its corrosive effect can harm the health of workers or the interior of the building and its contents. Unfortunately, this is not satisfactory. Worker health is impaired. Further, the interior of such buildings is an environment in which corrosion rapidly occurs. Attempting to solve this kind of pollution with atmospheric dilution is not satisfactory.

Layers of plastic balls or other acid-inert particles have also been attempted. The theory behind these floating layers is to form a circuitous path for the aerosol from the bursting bubbles — and thereby to attenuate the emission of mist to the environment. This does result in some mist reduction. The emitted aerosol to a limited extent condenses out on the floating objects and finds it way back to the bath. Unfortunately, acid mist or aerosol is still emitted in significant quantities. Therefore, while this expedient is commonly utilized, it does not constitute a complete solution to the problem.

An additional attempt to mitigate this problem has involved utilizing surfactant in the upper layers of the sulfuric acid bath. The theory is that the reduced surface tension of the acid solution will retard the incidence of bubble formation. While this works only to a limited extent, it has a severe drawback.

It will be remembered that the electrolytic solution is circulated through the bath on a continuous basis. When the solution leaves the bath, it goes through a solvent extraction process which enriches the copper content of the solution so that it can be returned to the tank for further electrolysis. This solvent extraction process is a precise, two phase chemical process in which contaminating surfactant cannot be tolerated. Simply stated, no matter how elaborate the precautions taken, sooner or later surfactants find their way into the solvent extraction process — and the process must be halted. Solution must be replaced, and production is

lost. Given that the placement of surfactants only results in a partial abatement of the problem, surfactant because of their interference with the solvent extraction side of the process are seldom used. Other attempts at solution of this problem have likewise been made. In Smith et al. U.S. Patent 4,668,353 issued May 26, 1987 entitled METHOD AND APPARATUS FOR ACID MIST REDUCTION, coalescing of aerosol is taught by providing surface limiting electrically inert masking device in which one portion is submerged in the electrolyte. The idea behind the device is to locally coalesce the mist and redeposit the coalesced acid back into the bath. Emission of aerosol still results.

In an alternate solution, partial "roofing" of the bath was attempted utilizing spanning eaves attached to the anode spanning to the cathode. Two effects occurred. First, the aerosol mist still escaped. Secondly, and during the reinsertion of the cathodes, sulfuric acid dripped from the underside of the eaves onto the harvested and freshly cleaned stainless steel cathodes. These cathodes, representing a significant investment of the total electrowinning process, were etched — especially where they extended above the bath. This being the case, this attempt was abandoned.

In short, a solution has not thus far been found for the vexing problem of the aerosol or mist of acid in electrowinning or electroplating processes.

SUMMARY OF THE INVENTION In a tank confined electrowinning process having circulated electroplating solution containing sulfuric acid, a multi-element cover system is applied below the electrode connections and above the surface of the electrolyte bath. This cover is evacuated in the interstices below the cover and above the bath at a rate exceeding the stoichiometric ratio causing any leakage to occur into the volume overlying the bath thereby preventing acid aerosol from escape.

The primary cover element constitutes dual hardness extruded polyvinyl chloride tapered anode caps cross bolted

through and fastened to opposite sides of the anodes by corrosion resistant fasteners. These anode caps each include an eave member spanning to the cathodes. These respective eaves are tapered and extend from a rigid portion of the extrusion fastened at the anode with sufficient span to form a substantially air tight seal with the cathodes immediately after the cathodes are freshly harvested and cleaned. The eaves on the underside preferably are sloped to and toward the anode. These eaves are sufficiently flexible to maintain a conformable seal at the inserted cathodes as well as to yield to allow the copper plated cathodes and their required edge strips to be both withdrawn and inserted. On the underside of the anode caps adjacent the ends of the eaves are so-called "drip lips" which protrude downward to and toward the bath. When the cathodes are inserted, the eaves flex downward toward the cathode. These drip lips then cause the sulfuric acid coalesced on the underside of the eaves of the anode caps to fall into the bath before reaching the cathode to avoid etching of the stainless steel of the freshly cleaned cathodes. At the respective tank sides normal to the plane of the anodes, a system of shingle-like overlapping flexible plastic strips form a substantially airtight seal to the tank sides and yet permit necessary insertion and withdrawal of the anodes. At the respective tank ends, covers are provided at both the electrolyte inlets and outlets. A ventilation exhaust system is communicated under the cover, preferably at the tank ends. This required ventilation system evacuates the underside of the resulting cover at a rate exceeding the stoichiometric ratio (preferably by a margin of 10 times) to acid mist and aerosol extraction apparatus which preferably constitute scrubbers. Thus, inevitable leakage of the resultant multi-component cover below the electrodes and above the acid bath occurs from the exterior of the cover into the ventilation evacuated interstices between the cover and bath. There results a cover system for the complete attenuation of acid mist in conventional electrowinning tank house installations, either on a retro-fit or new installation application.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a top plan view of an electrowinning tank for the reduction of copper by electrolysis broken away in the medial portion of the tank illustrating the multi-element cover and connected ventilation;

Fig. IB is a side elevation section taken to expose an anode illustrating the support and electrical connection of the electrodes above the bath surface with the multi-element cover of this invention disposed between the electrical connections and the bath surface;

Fig. 2 is a side elevation taken at the electrode cover elements of this invention, the cover elements here being shown fastened to both sides of an anode and bridging out into conforming substantial air tight contact with adjacent cathodes;

Fig. 3 is a side elevation section of the electrode cap of this invention with a dual hardness extrusion including a substantially rigid member for fastening to the electrode and a tapered flexible member for extending to an adjacent electrode, the construction here being of a cap for preferable attachment to an anode with a downward protruding lip for preventing dripping of acid to an adjacent cathode;

Figs. 4A and 4B are respective side elevation and plan views of side-by-side anode caps illustrating overlapping flexible planar members at the side edges of the cap which are shown in the view of Fig. 4A providing a substantially air tight seal at the tank sides;

Figs. 5A and 5B are respective plan views and side elevations of the tank end cover illustrating the caps defining a plenum for the withdrawal of air with acid mist;

Fig. 5C is a detail at the end of the tank illustrating the last anode end cap in contact with the seal at the end of the tank;

Figs. 6A and 6B are details of the end tank cap construction taken with respect to Fig. 5A; and.

Fig. 7 is a system and process schematic illustrating how the multi-component roof system of this invention is connected to evacuating ventilation and a mist

disengagement device (here shown as a scrubber) so as to effectively confine acid mist pollution to a contained path between the interstices of the tank cover and the illustrated scrubber.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 7, electrowinning tank T having a series of electrodes including anodes A and cathodes C are placed within a bath of copper bearing sulfuric acid aqueous solution. Direct current is conventionally supplied by apparatus not shown producing plated metal (here copper) on cathodes C and producing an acid mist.

A multi-component roof system R is placed over the acid bath B. This roof system is below the supports and electrode electrical connections of the anodes A and cathodes C but above the surface of bath B. Thus, between the underside of the multi-component roof R and bath B there is defined a plenum P.

Plenum P is evacuated by ventilation to mist disengagement device X, here shown as a scrubber. Such evacuation occurs at a rate exceeding the so-called stoichiometric ratio of oxygen gas by-product produced relative to the plating occurring. By way of example, it is known that for each 63 pounds of copper plated, stoichiometrically about 180 cubic feet of oxygen gas are produced. By substantially exceeding this rate of ventilation exhaust, all gas and acid mist will be withdrawn.

It should be noted that in order to permit this rate of evacuation, the multi-component roof R must admit air from atmosphere. Air enters from above roof R into plenum P.

Having schematically set forth this invention, the detail may now be understood referring to the remaining Figures.

Referring to Figs. 1A and IB, tank T is illustrated having a sulfuric acid bath B and depending supported cathodes C and anodes A. Electrical connection to the respective anodes A and cathodes C are made through their respective supports 16, 18, and are conventional and therefore not shown.

Cathodes C include an edge strip 14 which confines copper plating to the faces of the stainless steel cathodes C; thus the plated cathode can be readily removed, cleaned and prepared, and thereafter returned. Tank T has a constant flow of solution passing therethrough. This being the case, solution is input at inlet I and output at outlet 0.

The multi-element roof R formed by this invention defines below the electrical connections to the electrodes and above the surface of bath B a plenum P (See Fig. IB) . In the preferred embodiment, this plenum P is evacuated by vents V to mist extractor or scrubber X (not shown in Fig. 1A) . Since this evacuation occurs at a rate exceeding the production of oxygen gas by the plating process (the so-called stoichiometric rate) , the multi-element roof R leaks from above roof R into plenum P.

The construction of the multi-element roof R can be described in detail. First, and with respect to Figs. 2 and 3, the electrode caps will be described. Secondly, and with respect to Figs. 4A and 4B, the connection of the multi¬ element roof R to the side of tank T will be described. Finally, and with respect to Figs. 5A - 5C and 6A - 6B, the end tank construction will be set forth.

Referring to Fig. 2, the main working elements of the multi-component roof R extending between cathodes C and anodes A can be seen and understood. Anodes A are here shown with caps 30 extending to and forming a substantial air tight seal against cathodes C. The two cathodes there illustrated are shown with plated copper 22 at the bottom portion of the drawing shown in Fig. 2. Fastening of caps 30 is here effected by fasteners 32, which fasteners can be corrosion resistant bolt and nut fasteners.

It goes without saying that tank T, multi-element roof R, caps 30, and fasteners 32 are all constructed of non- corrosive materials. Polyvinyl chloride is suitable for roof R, caps 30, and fasteners 32. Likewise, fastening — as for example by clipping and the like — can occur.

The particular cap 30 here illustrated is designed to fit to the anode A. The reader will understand that variations of this design can include fitting the cap to cathode C or to both cathode and anode. What is important is that the electrode caps 30 utilized be capable of retro-fit and permit the substantially unobstructed removal and insertion of all of the electrodes - both anodes A and cathodes C — as necessary for carrying out the electrowinning process. Turning to Fig. 3, an electrode cap 30 is illustrated. This is a polyvinyl chloride extrusion including a lower rigid member 40 having spaced apart bores 42 that enable mounting by bolt and nut fasteners 32 to corresponding spaced apart bores on anode A. An upper flexible and tapered member 44 spans outwardly from cap 30 to tapered end 46. This tapered member 44 has undersurface 47 normally sloped away from cathode C toward supporting anode A.

Underside 47 of cap 30 includes a continuous ridge 48. The purpose of ridge 48 is to divert liquid acid coalescing from acid mist within plenum P from passing along undersurface 47 and onto a cathode C passing adjacent tapered end 46. This function can be more clearly understood once the dimension and flexibility function of flexible member 44 is understood. Regarding the dimension of flexible member 44, it is always of a length to permit a substantially air tight seal with an adjacent cathode C. This requirement effectively defines the span of the member.

Regarding the flexibility of flexible member 44, it must be flexible enough to allow plated cathode C with copper 22 to be with drawn. Further, sufficient flexibility must be provided to allow required cathode edge strips 14 (See Fig. IB) and any electrode spacers utilized between anode A and cathode C to pass. It will be understood that when an adjacent electrode — here a cathode — is inserted, bending downward of undersurface 47 will occur. It is at this time ridge 48 dislodges coalesced acid.

It will be understood that ridge 48 and end 46 will admit of variation. Any slope or structure which can prevent dripping of the coalesced acid onto the adjacent or attached electrode is intended to be covered. At the same time, it will be understood that the roof components including cap 30 are not air tight. It is actually preferred to have a constant and substantial air leakage from atmosphere to plenum P to insure isolation of the acid aerosol. Referring to Fig. 4A, it will be seen that the anode caps 30 are completed by a spacer 50 that extends between rigid members 40. Spacer 50 occupies the interval between the depending anode A and the sides of tank T. Thus, anode caps 30 will be understood to form in conjunction with the top of the anodes A and the top of the cathodes C, a continuous multi-element roof defining plenum P between the top of bath B and the underside of roof R.

With respect to the complete multi-element cover extending over tank T, this leave two areas unaccounted for. Those areas are the tank T sides and the tank T ends. It is to be understood that the coverage of these areas is required.

Referring to Figs. 4A and 4B, the covering to the tank T sides is easily understood. Referring to Fig. 4B, it will be seen that semirigid inert and flexible pads 60 are fastened to the respective ends 59 of electrode caps 30. These flexible pads have two important dimensions.

First, the dimension of pads 60 axially of the tank T is selected so that the pads 60 overlie one another like shingles on a roof. Unlike shingles on a roof, the particular order of overlap is not important, as the particular multi¬ element roof here shown "leaks" from the outside to the inside.

Secondly, the dimension of the pads 60 in a dimension measure across tank T is such that the pads cantilever into contact at the sides 61 of tank T. Thus, when anode A are lowered into tank T, and upward overlap 62 such as that shown in Fig. 4A occurs. Thus it will be understood that

the multi-element roof is substantially complete with respect to the tank sides.

Referring to Fig. 5A and 5B, tank roof end member 69 can be understood. An outlet cover 70 — which is conventional is shown. A cover 71 spans the tank T end and includes an end dam 74. Holes 72 provide for connection of exhaust vents V, providing the preferred plenum P discharge for this invention. Suitable overlap and fitting to tank T sides and ends is provided by conventional overlaps along cover 70.

Referring to Fig. 5C, it will be seen that end dam 74 depends downward below bath B. End tank anodes A span outward and contact end dam 74 much in the manner that they would contact an adjacent cathode C. Referring to Figs. 6A and 6B, it will be understood that end dams 74 are provided with spanning axial gussets 80, cross gussets 82 and an overhead seal strip 84. Strip 84 fits against cover 71 in overlap to substantially seal tank roof end member 69. It will be understood that the construction of this invention may vary from the preferred detail set forth herein. Specifically, electrode caps can be attached to the cathode. Likewise, the construction of the multi-element roof R can vary widely at tank T sides and ends to accommodate various tank and electrode arrays.